The invention relates to a method and apparatus for continuous cellulose digestion that does not require the use of screens in the digesting zone. In present installations, clogged screens in the digesting zone are a major cause of downtime for continuous digestors. Liquid usually passes through the screens under high pressure which has a tendency to cause cellulose particles to adhere to the screen faces, building up and causing complete blockage of liquid flow through the screens. If such blockage is not eliminated, then uneven treatment of the cellulose material may result. The raw product that is treated in modern day digestors often contains a large amount of leaves, bark, etc., and therefore has a great tendency to clog conventional digesting zone screens.
According to the present invention, the screens of prior art digestors -- such as shown in U.S. Pat. Nos. 3,007,839, 3,200,032, and 3,298,899 -- are eliminated, separation of liquid and cellulose fiber chips taking place in a clarifying portion at the top of the digestor, and thus there obviously are no hangups due to screen clogging, while effective liquid-chips separation is effected. The cylindrical impregnation zone vessel of the continuous digestor according to the present invention is of smaller diameter and relatively longer than conventional continuous digestor impregnation zones, and extends downwardly into the digesting zone to a point well below the liquid outlets at the top of the digesting zone. The cylindrical casing for the digesting zone is of significantly larger diameter than the diameter of the impregnation zone (the area of the impregnation zone being approximately 1/4 - 1/3 the area of the digesting zone), and is liquid filled and has digesting liquid outlets disposed at the top thereof, above the lower end of the impregnation zone vessel. The inlet for the digesting liquor must be sufficiently below the end of the impregnation zone casing so that there is adequate, even distribution of the heated liquid through the chips column, a countercurrent flow to the downward movement of the chips being established. This normally requires the inlet for the digesting liquor be located a distance below the impregnation zone casing of at least the radius of the digesting vessel. The chips column effectively acts as a heat distributor.
When the chips move downwardly from the impregnation zone into the digesting zone, because of the increase in diameter of the container, they tend to flow generally outwardly and downwardly, and establish a generally conical surface top over the area of the digesting zone. This surface acts to separate the liquid flowing upwardly therethrough at slow speed (approximately 5 ft./min., whereas a velocity of approximately 50 ft./min. would be necessary to entrain the chips particles with the liquid for upward flow therewith) from the chips, and thus forms a clarifying zone in the upper portion of the digestor zone above the bottommost portion of the impregnation zone vessel. Since the chips are already separated from the liquid by the time the liquid reaches the outlets, there is no need to provide screens at the outlets at the top of the digesting zone.
In order to further ensure that no chips flow through the liquid outlets at the top of the digesting zone, a pair of screened portions connected to suction means are provided below the liquid outlet, but above the chip column, never being in contact therewith. The suction means for each of the screened portions are alternately actuated, to suck particles adjacent thereto into contact with the screen face, and then to release the suction to allow the particles to fall downwardly back into the chips column.
In addition to providing the advantage of no screens, the present invention also results in lower compaction and a freer movement of the column, which in turn allow superior counter-current washing in the washing zone, and assists in cleaning the screens therein. Also, although screenless washing, such as disclosed in U.S. Patent application Ser. No. 423,812 of Richter et al., filed Dec. 11, 1973, now abandoned, in a separate and distinct zone may be utilized, there is really to need therefor, washing in the continuous digestor vessel itself being practical with reduced chances of the washing zone screens becoming clogged.
It is the primary object of the present invention to provide for cellulose digestion without utilizing screens for digesting liquor recirculation. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.